Pigment with day-light fluorescence

ABSTRACT

A pigment, especially a yellow to red emitting luminescent material, with a host lattice of the nitridosilicate type M x Si y N z :Eu, wherein M is at least one of an alkaline earth metal chosen from the group Ca, Sr, Ba and wherein z=⅔x+{fraction (4/3)}y.

This application is a U.S. National Phase application under 35 U.S.C.371 of International Application PCT/EP00/12047 filed Nov. 30, 2000published in English).

TECHNICAL FIELD

This invention relates to a Pigment with day-light fluorescence and moreparticularly, but not exclusively to a pigment absorbing blue to greenlight and emitting fluorescence within the yellow to red spectral regionunder excitation by daylight or by an artificial light source. Furtherabsorption in other spectral regions is possible, especially in the UV.More specifically, such a pigment can be used as a phosphor for lightsources, especially for Light Emitting Diodes (LED) or electrical lamps.The pigment belongs to the class of rareearth activated siliconnitrides.

BACKGROUND ART

For Eu²⁺-doped material normally UV-blue emission is observed (Blasseand Grabmeier: Luminescent Materials, Springer Verlag, Heidelberg,1994). Several studies show that also emission in the green and yellowpart of the visible spectrum is possible (Blasse: Special Cases ofdivalent lanthanide emission, Eur. J. Solid State Inorg. Chem. 33(1996), p. 175; Poort, Blokpoel and Blasse: Luminescence of Eu²⁺ inBarium and Strontium Aluminate and Gallate, Chem. Mater. 7 (1995), p.1547; Poort, Meijnhoudt, van der Kuip, and Blasse: Luminescence of Eu²⁺in Silicate host lattices with Alkaline earth ions in a row, J. Alloysand Comp. 241 (1996), p. 75). Hitherto, red Eu²⁺ luminescence isobserved only in some exceptional cases, such as in alkaline earthsulphides and related lattices of the rock-salt type (Nakao,Luminescence centers of MgS, CaS and CaSe Phosphors Activated with Eu²⁺Ion, J. Phys. Soc. Jpn. 48(1980), p. 534), in alkaline earththiogallates (Davolos, Garcia, Fouassier, and Hagenmuller, Luminescenceof Eu²⁺ in Strontium and

Barium Thiogallates, J. Solid. State Chem. 83 (1989), p. 316) and insome borates (Diaz and Keszler; Red, Green, and Blue Eu²⁺ luminescencein solid state Borates: a structure-property relationship, Mater. Res.Bull. 31 (1996), p. 147). Eu²⁺ luminescence in alkaline-earth siliconnitrides has hitherto only been reported for MgSiN₂:Eu (Gaido,Dubrovskii, and Zykov: Photoluminescence of MgSiN₂ Activated byEuropium, lzv. Akad. Nauk SSSR, Neorg. Mater. 10 (1974), p. 564;Dubrovskii, Zykov and Chernovets: Luminescence of rare earth ActivatedMgSiN₂, lzv. Akad. Nauk SSSR, Neorg. Mater. 17 (1981), p. 1421) andMg_(1-x)Zn_(x)SiN₂:Eu (Lim, Lee, Chang: PhotoluminescenceCharacterization of Mg_(1-x)Zn_(x)SiN₂:Tb for Thin FilmElectroluminescent Devices Application, Inorganic and OrganicElectroluminescence, Berlin, Wissenschaft und Technik Verlag, (1996), p.363). For both Eu²⁺ luminescence in the green and green/blue part of thespectrum was found.

New host lattices of the nitridosilicate type are based on a threedimensional network of cross-linked SiN₄ tetrahedra in which alkalineearth ions (M=Ca, Sr and Ba) are incorporated. Such lattices are forexample Ca₂Si₅N₈ (Schlieper and Schlick: Nitridosilicate 1,Hochtemperatursynthese und Kristallstruktur von Ca₂Si₅N₈, Z. anorg.alig. Chem. 621, (1995), p. 1037), Sr₂Si₅N₈ and Ba₂Si₅N₈ (Schlieper,Millus and Schlick: Nitridosilicate II, Hoch-temperatursynthesen undKristallstrukturen von Sr₂Si₅N₈ and Ba₂Si₅N₈, Z. anorg. alig. Chem. 621,(1995), p. 1380), and BaSi₇N₁₀ (Huppertz and Schnick: Edge-Sharing SiN₄tetrahedra in the highly condensed Nitridosilicate BaSi₇N₁₀, Chem. Eur.J. 3 (1997), p. 249). The lattice types are mentioned in Table 1.

Sulfide based phosphors (e.g. earth alkaline sulfides) are lessdesirable for lighting applications, especially for LED applications,because they interact with the encapsulating resin system, and partiallysuffer from hydrolytic attack. Red emitting Eu²⁺ activated berates showalready temperature quenching to a certain degree at the operatingtemperature of LEDs.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of this invention to obviate thedisadvantages of the prior art. It is another object of the invention toprovide a pigment for day-light fluorescence. It is a further abject toprovide a yellow to red emitting luminescent material which is excitableat wavelengths around 200 to 500 nm, preferably 300 to 500 nm, togetherwith high chemical and thermal stability.

Especially high stability up to at least 100° C. is highly desirable forLED applications. Their typical operation temperature is around 80° C.

These objects are accomplished by the characterising features ofclaim 1. Advantageous embodiments can be found in the dependant claims.

The new pigments show at least absorption within the blue-green spectralregion. Furthermore they show fluorescent emission under absorption.Those Eu²⁺-doped luminescent materials show emission within the yellowto red spectral region, especially long wavelength red, orange or yellowemission. These pigments are based on alkaline-earth silicon nitridematerial as hostlattices. They are very promising, especially for LEDapplications, when used as phosphors. Hitherto white LEDs were realisedby combining a blue emitting diode with a yellow emitting phosphor. Sucha combination has only a poor colour rendition. A far better performancecan be achieved by using a multicolour (for example red-green-blue)system. Typically the new material can be used together with agreen-emitting (or yellow-emitting) phosphor, for examplestrontiumaluminate SrAl₂O₄:Eu²⁺, whose emission maximum is around 520nm.

In detail, the new Pigment with day-light fluorescence uses a hostlattice of the nitridosilicate type M_(x)Si_(y)N_(z):Eu, wherein M is atleast one of an alkaline earth metal chosen from the group Ca, Sr, Baand wherein z=⅔x+{fraction (4/3)}y. The incorporation of nitrogenincreases the proportion of covalent bond and ligand-field splitting. Asa consequence this leads to a pronounced shift of excitation andemission bands to longer wavelengths in comparison to oxide lattices.

Preferably, the pigment is of the type, wherein x=2, and y=5. In anotherpreferred embodiment, the pigment is of the type, wherein x=1, and y=7.

Preferably, the metal M in the pigment is strontium because theresulting phosphor is emitting at relatively short yellow to redwavelengths. Thus the efficiency is rather high in comparison to most ofthe other elected metals M.

In a further embodiment the pigment uses a mixture of different metals,for example Ca (10 atom.-%) together with Ba (balance), as component M.

These materials show high absorption and good excitation in the UV andblue visible spectrum (up to more than 450 nm), high quantum efficiencyand low temperature quenching up to 100° C.

It can be used as a pigment for coloring goods or as a phosphor forluminescence conversion LEDs, especially with a blue light emittingprimary source together with one or more other phosphors (red andgreen).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Diffuse reflection spectra of undoped Ba₂Si₅N₈ and Ba₂Si₅N₈:Eu;

FIG. 2: Diffuse reflection spectra of undoped BaSi₇N₁₀ and BaSi₇N₁₀:Eu;

FIG. 3: Emission spectrum of Ba₂Si₅N₈:Eu;

FIG. 4: Emission spectrum of BaSi₇N₁₀:Eu;

FIGS. 5-7: Emission spectrum of several embodiments of Sr₂Si₅N₈:Eu;

FIG. 8: Emission spectrum of Ca₂Si₅N₈:Eu.

DETAILED EMBODIMENTS

Eu₂O₃ (with purity 99,99%), or Eu metal (99,99%), Ba metal (>99%); Srmetal (99%), Ca₃N₂ (98%), or Ca powder (99,5%) and Si₃N₄ (99,9%) wereused as commercially available starting materials. Ba and Sr werenitrided by firing at 550 and 800° C. under a nitrogen atmosphere.Subsequently, Ca₃N₂ or nitrided Ba, Ca or Sr were ground in a mortar andstoichiometrically mixed with Si₃N₄ under nitrogen atmosphere. TheEu-concentration was 10 atom.-% compared to the alkaline earth ion. Thepowdered mixture was fired in molybdenum crucibles at about 1300-1400°C. in a horizontal tube furnace under nitrogen/hydrogen atmosphere.After firing, the powders were characterised by powder X-ray diffraction(Cu, Kα-line), which showed that all compounds had formed.

The undoped Ba₂Si₅Na₈, Ca₂Si₅N₈ and BaSi₇N₁₀ are greyish-white powders.These undoped rare-earth activated silicon nitrides show high reflectionin the visible range (400-650 nm) and a strong drop in the reflectionbetween 250-300 nm (FIGS. 1 and 2). The drop in reflectance is ascribedto host-lattice absorption. The Eu-doped samples are orange-red, exceptfor BaSi₇N₁₀:Eu which is orange-yellow (Table 1). The strong colorationis unique for Eu²⁺-doped rare-earth activated silicon nitrides and makethese material interesting orange-red pigments. A typical example of areflection spectrum of Ba₂Si₅N₈:Eu shows that the absorption due to Euis superposed on the hostlattice absorption and extends up to 500-550 nm(FIG. 1). This explains the red-orange colour of these compounds.Similar reflection spectra were observed for Sr₂Si₅N₈:Eu andCa₂Si₅N8:Eu.

For BaSi₇N₁₀:Eu the absorption of Eu is less far in the visible part(FIG. 2), which explains the orange-yellow colour of this compound.

All samples show efficient luminescence under UV excitation withemission maxima in the orange-red part of the visible spectrum (seeTable 1). Two typical examples of emission spectra can be seen in FIGS.3 and 4. They show that the emission is at extremely long wavelengths(for Eu²⁺ emission) with maxima up to 660 nm for BaSi₇N₁₀:Eu (FIG. 4.).Excitation bands are observed at low energy which is the result of acentre of gravity of the Eu²⁺ 5d band at low energy and a strongligand-field splitting of the Eu²⁺ 5d band, as can be expected for N³⁻containing lattices (van Krevel, Hintzen, Metselaar, and Meijerink: LongWavelength Ce³⁺-luminescence in Y—Si—O—N Materials, J. Alloys and Comp.168 (1998) 272).

Since these materials can convert blue into red light due to low-energyexcitation bands, they can be applied in white light sources, forexample based on primarily blue-emitting LED's (typically GaN or InGaN)combined with red, yellow and/or green emitting phosphors.

TABLE 1 Emission Compound Crystal structure Color Maximum (nm)*Ca₂Si₅N₈:Eu Monoclinic Orange-Red 600 to 630 Sr₂Si₅N₈:Eu OrthorhombicOrange-Red 610 to 650 Ba₂Si₅N₈:Eu Orthorhombic Orange-Red 620 to 660BaSi₇N₁₀:Eu Monoclinic Orange-Yellow 640 to 680 *depending on theconditions for preparation and concentration of the activator; typicalvalues for Eu-concentration may vary between 1 and 10% compared to thealkaline-earth ion M

These emission maxima are unusually far in the long wavelength side. Aspecific example is a phosphor of the type Sr_(1.8)Eu_(0.2)Si₅N₈. Itsemission spectrum is shown in FIG. 5.

Another embodiment for realising M is the use of Zn. It can replace Ba,Sr or Ca fully or partially.

A further embodiment for replacing Si fully or partially is Ge. Anconcrete embodiment is Sr_(1.8)Eu_(0.2)Ge₅N₈.

Some further specific examples were investigated:

The preparation conditions and optical properties of the red emittingphosphor Sr₂Si₅N₈:Eu²⁺ were investigated. Optimisation showed a quantumefficiency of about 70%. The emission is tuneable between 610 and 650nm, depending on the Eu²⁺ concentration in the sample and the heatingconditions. The absorption at 400 nm and 460 nm is high (reflection ofonly 15-40%) and the temperature quenching of the luminescence at 80° C.is low (only 4%). The particle size of the phosphor is without millingbelow 5 μm. These properties make this phosphor very interestingespecially for application in both the UV and blue LED.

For the nitride synthesis, the starting materials are Si₃N₄ (99,9%(mainly α-phase), Alfa Aesar), Sr metal (dendritic pieces 99,9%, AlfaAesar) and Eu₂O₃ (4N). The Sr metal has to be nitrided and in case oneuses instead of Eu₂O₃ Eu metal, this has also to be nitrided.

The Sr metal is milled by hand in an agath mortar in an argon gloveboxand nitrided at 800° C. under N₂. This results in a nitration over 80%.

After remilling, the nitrided metal, together with Si₃N₄ and Eu₂O₃, ismilled and mixed by hand again in the glovebox. The heating of thismixture has typically the following parameters:

18° C./min to 800° C.

5 h at 800° C.

18° C./min to T_(e)nd (1300-1575° C.) 5 h at T_(end)(1300-1575° C.)

H₂(3.75%)/N₂ 400 l/h

Ca₂Si₅N₈:Eu²⁺ samples were made with Ca₃N₂ as starting material.

An overview of all the samples is given in table 1. Typically, thesamples were first heated at 800° C., and then they were heated a secondtime in the same cycle at elevated (1300-1600° C.) temperatures. Thesamples were then milled (mill under air), sieved and measured.

TABLE 1 parameters of heating cycles of (Ca,Sr)₂Si₅N₈:Eu²⁺ samples Eu²⁺Time Temp. Time Temp. Code Ca/sr (%) 1 (h) 1 (° C.) 2 (h) 2 (° C.) EC/HU31/00 Ca 10 5 800 5 1400 EC/HU 42/00 Ca 1 5 800 5 1565 EC/HU 41/00Ca0.4Sr1.4 10 5 800 5 1565 EC/HU 62/00 Sr 1 5 800 5 1400 EC/HU 63/00 Sr2 5 800 5 1400 EC/HU 64/00 Sr 3 5 800 5 1400 EC/HU 65/00 Sr 5 5 800 51400 EC/HU 66/00 Sr 8 5 800 5 1400 EC/HU 67/00 Sr 10 5 800 5 1400

The samples that are obtained after this heating show a color of deeporange for 10% Eu²⁺ containing Sr₂Si₅N₈ samples. With less Eu²⁺ thecolour is fainter. The Ca samples have a yellow-orange colour.

There is also another interesting feature: the powder particles are verysmall with an average particle size d₅₀ between about 0,5 and 5 μm, atypically value is d₅₀=1.3 μm. The small particle sizes are advantageousfor the processing of LEDs with luminescent material. For example theyallow a homogeneous distribution in the resin.

TABLE 2 Optical data of (Ca,Sr)₂Si₅N₈:Eu²⁺ samples Em. Refl. Refl. Eu²⁺Max 400 460 QE Code Ca/Sr (%) (nm) (%) (%) (%) x y EC/HU Ca 10 619 12 1926 0.600 0.396 31/00 EC/HU Ca 1 603 47 58 37 0.555 0.435 42/00 EC/HUCa0.4 10 660 17 22 59 0.636 0.,354 41/00 Sr1.4 EC/HU Sr 1 609 53 58 700.602 0.393 62/00 EC/HU Sr 2 618 43 48 73 0.615 0.381 63/00 EC/HU Sr 3621 36 41 72 0.622 0.374 64/00 EC/HU Sr 5 624 26 32 67 0.632 0.365 65/00EC/HU Sr 8 636 21 26 67 0.641 0.356 66/00 EC/HU Sr 10 644 17 22 64 0.6420.354 67/00

Concerning table 2 all samples were typically first heated in a firstcycle (for example 800° C. for 5 h), as already outlined above.

Included in table 2 are the position of the emission maximum, the meanwavelength, the reflection at 400 and 460 nm, the quantum efficiency andfinally the x and y colour coordinates.

From table 2 it can be derived that the pure Ca samples are not asfavourable as the Sr samples. It is surprising that the Sr-Ca compoundhas an emission wavelength that is larger than that of the pure Srcompound.

Specific examples are shown in FIGS. 6 to 8. FIG. 6 shows the energydistribution (in arbitrary units) and reflection (in percent) of sampleHU 64/00 (Sr₂Si₅N₈:Eu²⁺) having a proportion of 3% Eu and a quantumefficiency of 72%. FIG. 7 shows the energy distribution (in arbitraryunits) and reflection (in percent) of sample HU 65/00 (Sr₂Si₅N₈:Eu²⁺)having a proportion of 5% Eu and a quantum efficiency of 67%. FIG. 8shows the energy distribution (in arbitrary units) and reflection (inpercent) of sample HU 42/00 (Ca₂Si₅N₈:Eu²⁺) having a proportion of 1% Euand a quantum efficiency of 37%.

What is claimed is:
 1. Pigment with day-light fluorescence, with a hostlattice of the nitrodosilicate type M_(x)Si_(y)N_(z):Eu having SiN₄tetrahedra wherein M is at least one of an alkaline earth metal selectedfrom the group consisting of Ca, Sr, Ba, and Zn and said SiN₄ tetrahedraare cross-linked to a three-dimensional network in which alkaline earthmetal M ions are incorporated and wherein z=⅔x+{fraction (4/3)}y. 2.Pigment according to claim 1, wherein x=2, and y=5.
 3. Pigment accordingto claim 1, wherein x=1, and y=7.
 4. Pigment according to claim 1,wherein M is strontium.
 5. Pigment according to claim 1, wherein M is amixture of at least two metals of said group.
 6. Pigment according toclaim 1, wherein said pigment is absorbing within the blue to greenspectral region.
 7. Pigment according to claim 6, wherein said pigmentis fluorescent within the yellow to red spectral region.
 8. Pigmentaccording to claim 1, wherein M is at least one of an alkaline earthmetal selected from the group consisting of Sr, Ba and Zn alone or incombination with Ca and wherein z=⅔x+{fraction (4/3)}y.
 9. Pigment withday-light fluorescence, within a host lattice of the nitrodosilicatetype MxSiyNz:Eu, wherein M is at least one of an alkaline earth metalselected from the group consisting of Ca, Sr, Ba, Zn and whereinz={fraction (2/2)}x+{fraction (4/3)}y, and wherein x=2, and y=5. 10.Pigment with day-light fluorescence, within a host lattice of thenitrodosilicate type M_(x)Si_(y)N_(z):Eu, wherein M is at least one ofan alkaline earth metal selected from the group consisting of Ca, Sr,Ba, Zn and wherein z=⅔x+{fraction (4/3)}y, and wherein x=1, and y=7. 11.Pigment with day-light fluorescence, with a host lattice of thenitrodosilicate type M_(x)Si_(y′)Ge_(y″)N_(z):Eu (where y′+y″=y) havingSi₄ tetrahedra wherein M is at least one of an alkaline earth metalselected from the group consisting of Ca, Sr, Ba, and Zn and said SiN₄tetrahedra are cross-linked to a three-dimensional network in whichalkaline earth metal M ions are incorporated and wherein z=⅔x+{fraction(4/3)}y.
 12. A coloring pigment comprising the pigment according toclaim
 1. 13. A phosphor excited by light sources, said phosphor emittingwithin the yellow-to-red spectral region and comprising the pigmentaccording to claim 1.